Page 409 - IJB-10-1

International Journal of Bioprinting Droplets prepared by air-focused bioprinting

showed a general trend that the droplet diameter decreased predicted by fluid dynamic simulations was essentially
until it reached a plateau as the air flow rate increased, as consistent with that of experiments. For example, when
shown in Figure 2d. Although the three structures had the the air flow rate increased from 50 to 400 mL/min, the
same inner and outer diameters, the inward contraction droplet diameter decreased from ~200 to ~125 μm for both
design had smaller droplet size and size distribution, simulation and experiment results, as shown in Figure 3b.
which was attributed to the larger We number, the stable Similarly, the droplet diameter increased when the inner
and strong viscous force of focused air flow, and thus the nozzle diameter increased, according to both simulation
inward contraction design was adopted for the following and experiment results, as shown in Figure 3c. Therefore,
studies. When changing the size of the inner nozzle, the fluid dynamic simulations could provide relatively good
droplet diameter increased as the inner nozzle diameter predictions of experiment results.
increased, since the surface tension that held the liquid tip Since AFMDP used air as the continuous phase, the
to the nozzle increased, as shown in Figure 2e. system enabled the fabrication of monodisperse particles
Fluid dynamic simulations were performed to further by using droplets as templates and avoided the removal
investigate the droplet formation process within the of the oil phase after preparation. For example, droplets
microfluidic device. As the liquid flow tip was gradually of alginate hydrogel could be crosslinked into hydrogel
2+
raised, it eventually broke the nozzle into droplets, when F v particles in a collection bath with Ca cations, as shown in
+ G > F , as shown in Figure 3a. Fluid dynamic simulations Figure 4a. This was because divalent Ca cations could serve
2+
s
2+
adopted the same parameters of inner nozzle diameter, as crosslinkers, and each Ca cation could simultaneously
outer nozzle diameter, liquid viscosity, liquid density, bind to two carboxylic groups from two different alginate
liquid flow rate, air viscosity, air density, and air flow rate as molecules. Similar to droplets, hydrogel particles prepared
those of experiments, and the change of droplet diameter by AFMDP also had a narrow size distribution of 196 ±

Figure 3. Fluidic dynamic simulation of droplet formation under focused air flow. (a) Snapshots showing the formation process of liquid droplets under
focused air flow. The red color denoted liquid; the green color denoted the liquid/air interface; the blue color denotes air. Dependences of (b) droplet
diameter versus air flow rate and (c) droplet diameter versus inner capillary diameter obtained by simulations and experiments. Fluid dynamic simulations
adopted same parameters of inner nozzle diameter, outer nozzle diameter, liquid viscosity, liquid density, liquid flow rate, air viscosity, air density, and air
flow rate as those of experiments.

Volume 10 Issue 1 (2024) 401 https://doi.org/10.36922/ijb.1102